Aromatic polyesters generally represented by polyalkylene terephthalates, such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), are well known engineering resins that exhibit desirable chemical resistance, gas barrier and heat-resistance properties making them candidates for use as containers for food or chemicals. In this connection, sheets of aromatic polyesters may be thermoformed (e.g., vacuum-formed or pressure-formed) into suitable containers.
Although PET containers having excellent transparency may be obtained by thermoforming an unoriented amorphous PET sheet at temperatures which do not promote crystallization, the heat-resistance properties of such a container are less than desirable. For example, thermoformed transparent containers of amorphous PET typically deform when subjected to environments heated to 80.degree. C. or more. In addition, when used in such high-temperature environments, the amorphous PET tends to crystallize thereby promoting shrinkage and/or a decrease in the container's transparency. As a result, the use of containers formed of amorphous PET resin are typically limited to low temperature environments.
To overcome the problems discussed immediately above, transparent PET sheets which have been crystallized by biaxial orientation have been used to form containers. However, thermoforming crystallized sheets of PET is relatively difficult owing to the biaxial orientation.
The temperature at which PET sheets are thermoformed may be increased so as to enhance the heat-resistance and mechanical strength properties of the resulting container. However, an increased thermoforming temperature brings the risks of promoting crystallization of the PET resin which, in turn, results in a more opaque ("whitened") appearance. Lowering the crystallization of PET resin by incorporating a comonomer into the polymeric chain could be envisioned, but such a proposal is typically not satisfactory for containers used in high temperature environments since the incorporated comonomer also tends to decrease the resin's mechanical characteristics, especially its heat-resistance properties.
Polybutylene terephthalate (PBT) resin, on the other hand, is generally difficult to form into a transparent sheet by quenching owing to its relative high rate of crystallization. Moreover, even if a transparent PBT sheet could be obtained, the PBT sheet exhibits poor thermoforming properties which typically prevents it from being shaped by thermoforming techniques into a container.
Copolymers prepared by incorporating other constituent comonomeric units into the PBT polymer chain tend to lower the crystallinity of the resin so that it exhibits enhanced transparency properties. However, this transparency tends to degrade over time to an opaque appearance in high temperature environments due to increased PBT crystallization which is promoted by such heat. Amorphous PBT copolymers, however, do not exhibit the desirable properties which are inherent with PBT resins generally, such as mechanical strength and heat-resistance properties, and therefore are not good candidates for forming containers to be used in high temperature environments.
What has been needed, therefore, is an aromatic polyester resin which exhibits exceptional mechanical and heat-resistance properties due to its highly crystalline structure, but also is highly transparent which could be thermoformed into containers useful in high temperature environments. It is towards fulfilling such a need that the present invention is directed.
Broadly, the present invention relates to processes for forming heat-resistant and transparent containers by subjecting a sheet made from a polypropylene terephthalate (PPT) resin to specific treatment conditions, and to the resulting heat-resistant and transparent containers thereby produced.
More specifically, the present invention is embodied in a process for producing a transparent and heat-resistant container by forming a sheet from a molten polyester resin having ester units derived from 1,3-propanediol and terephthalic acid or ester-forming derivatives thereof, quenching the sheet so as to form a solid resin sheet having low crystallinity, followed by aging and shaping the sheet by thermoforming under specified conditions so that the resulting container is highly (i.e., at least 50%, and more preferably at least 80%) crystalline. The polyester that is used in the practice of this invention is one comprised of ester units derived from 1,3-propanediol and ester-forming derivatives thereof and terephthalic acid in an amount of at least 80 mole% based on the entirety of the repeating units in the polymer chain.
Further aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.